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1 skier caught, not buried or injured
A brief period (4 - 6 hours) of cold, clear weather at higher elevations on February 4 lead to the formation of thin (1/2 - 1 mm) pockets of surface hoar subsequently buried under 8+ inches of new snow. Temperatures in the valley were moderate to warm, but surface hoar was preserved in patches on North to Northeast aspects in the mountains. On February 6 there had been 1 1/2 ft. of snow since February 1. About 5" more fell that night. There were occasional flurries on February 7 and gusty SW winds (25 - 40mph) on the ridges. The total snowpack was about 12 feet. There was no significant temperature gradient in the snowpack, and all but the new snow was well consolidated and stable. The temperature was 28F at noon.
T. and I (S.) discussed avalanche hazard potential and went out for some afternoon skiing. We headed for the ridge between the Bonny Lakes and the N. Fork of the Imnaha drainage. The ridge runs from the NE to the SW. We approached the NE facing aspect. T. chose an up-route among trees, which seemed safe enough for conditions.
The slope was 32 - 38 degrees. About half way up, we stopped and dug a snow pit 4 1/2 feet deep. Nothing failed with a compression test. A shovel shear test yielded a moderate to hard shear about 2 1/2 feet down. It sheared above a thin ice crust. Below the crust the snow was well consolidated, finger to pencil density. Above the crust was a little over 1 ft. of 4F to 1F snow, with another foot of fist to 1F snow above. Temperatures had been balmy for the better part of a week in town, and I had been skiing in the rain on Wing Ridge at 7800' 5 days earlier, so I expected a well consolidated stable snowpack. We both agreed that the snowpack seemed stable and that we could ski this aspect safely. However, we noted several snow "pillows" and the convex slope feature on the upper 1/4 of this aspect.
We continued to the top of the ridge and encountered gusty winds and blowing fog and snow from the SW. We intended to take a few runs on the SW facing slope before returning via the north slope. However, the south aspects were breakable crust, so we returned to the powder covered NE aspect. Shortly, T. was skiing downhill next to our up track. The immediate descent was not much over 200 feet. When he was down, I started down on the skier's left of his track. I had a more open shot with a slight convex breakover, which made me a little nervous, but I did not want to ski through the up track. I had made about 6 turns when I noticed that the snow felt loose and some powder was running on top.
I thought about heading toward the trees and was about to cross T.'s track when the fracture broke above me. I had a glimpse of the snow moving and then was falling with it. I rolled and got a mouthful of snow, but got upright again and grabbed a tree branch. The snow continued under me. T. yelled my name, I called back that I was safe. T. witnessed the slide but did lose sight of me during the event behind small trees in the slide path. I collected one pole from the surface. T. and I spent 2 hours probing with shovels for my other pole and skis. They weren't far from where I ended up and they were buried less than a foot deep.
The avalanche was classified as an SS-AS-2-0 (Soft slab - skier triggered small slide - old snow bed surface). After the slide, we climbed back up to examine the crown fracture. With close scrutiny we saw buried surface hoar, appx. 1/2 - 1 millimeter thick. The slab was very soft - fist to 4 finger hardness. Slab thickness varied from around 8 - 20 inches along the crown fracture. Within the soft slab, the newest snow was damp powder and it was on top of a light wind slab. The bed surface was not much harder, 4 finger to 1 finger. R. dug a pit on Feb. 9 near our snow pit / uptrack and confirmed that the buried surface hoar was not present there. R. found the surface hoar still present in the crown / flank, and obtained an easy shovel shear at the crown. The distance between our uptrack / test pit and the closest edge of the fracture was appx. 100 ft. At the time T. and I gained the ridge that day, no natural slides were noted in the Imnaha drainage (the upper part of the north facing aspect of the ridge to the south). Many slides were noted there 2 days later. We did not see any other slides in the Bonny Lakes basin that day or the following day when we had good visibility.